In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drivetrains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drivetrain, but must do so while maintaining reasonable levels of performance, range, reliability, safety and cost.
The most common approach to achieving a low emission, high efficiency car is through the use of a hybrid drivetrain in which an internal combustion engine (ICE) is combined with one or more electric motors. While hybrid vehicles provide improved gas mileage and lower vehicle emissions than a conventional ICE-based vehicle, due to their inclusion of an internal combustion engine they still emit harmful pollution, albeit at a reduced level compared to a conventional vehicle. Additionally, due to the inclusion of both an internal combustion engine and an electric motor(s) with its accompanying battery pack, the drivetrain of a hybrid vehicle is typically more complex than that of either a conventional ICE-based vehicle or an all-electric vehicle, resulting in increased cost and weight. Accordingly, several vehicle manufacturers are designing vehicles that only utilize an electric motor, thereby eliminating one source of pollution while significantly reducing drivetrain complexity.
In order to achieve the desired level of wheel torque in an electric vehicle (EV), the powertrain is typically coupled to the wheels using a suitable gear reduction assembly. Under normal driving conditions this approach provides a highly efficient drive train. Unfortunately under certain abnormal driving conditions that result in a loss and then a regain of traction, this approach can lead to an excessive load being placed on the drive train. This load is due to the rapid decrease in motor speed that occurs when the wheels regain traction. Depending upon the length of time during which traction is lost, when traction is regained the deceleration related inertia torque can generate excessively large torque loads that can damage various drive train components. Although the drive train can be designed to withstand these torque loads, this approach leads to a greatly over designed, expensive and heavy drive train. Accordingly, what is needed is a system that can limit drive train load to a useful range. The present invention provides such a system.